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Issue
A&A
Volume 519, September 2010
Article Number A59
Number of page(s) 3
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201014429
Published online 14 September 2010
A&A 519, A59 (2010)

The mid-UV population of the nucleus and the bulk of the post-merger NGC 3610

L. M. Buson

INAF Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy

Received 15 March 2010 / Accepted 3 August 2010

Abstract
Context. The very center ( $r\ll r_{\rm e}$) of NGC 3610, a clearly disturbed giant elliptical that is generally assumed to be a post-merger remnant, appears dominated in the mid-UV (2500-3200 Å spectral region) by a stellar population that is markedly different from that dominating the bulk of its stellar body.
Aims. We make use of the mid-UV spectra of NGC 3610 as seen through tiny ($\sim $1'') and large ( $10''\times20''$) apertures as a diagnostic population tool.
Methods. We compare archive IUE/LWP large aperture and HST/FOS UV data of NGC 3610.
Results. The strength of mid-UV triplet (dominated by the turnoff population) shows a remarkable drop when switching from the galaxy central arcsec (FOS aperture) to an aperture size comparable to $\sim $0.5 $r_{\rm e}$ (IUE).
Conclusions. The sub-arsec (mid)-UV properties of this galaxy involved in a past merger reveal a central metal enrichment which left the bulk of its pre-existing population intact.

Key words: galaxies: general - galaxies: elliptical and lenticular, cD - galaxies: individual: NGC 3610 - galaxies: interactions

1 Introduction

The E5 galaxy NGC 3610 appears peculiar in many respects. It has the richest fine structure of all ellipticals discussed by Seitzer & Schweizer (1990), as well as a warped central disk identified by Scorza & Bender (1990) by means of their photometric decomposition and later confirmed by means of HST by Whitmore et al. (1997). In addition, its anomalous (B-V) color (e.g. Goudfrooij et al. 1994) is an indicator of recent star formation. At the same time the galaxy is encircled by a globular cluster (GC) system that shows evidence of a past merger of metal-rich disk-disk galaxies (Goudfrooij et al. 2007).

Because the epoch of the major merger which formed NGC 3610 is estimated to be $4 \pm 2.5$ Gyr ago (Strader et al. 2004), it is an ideal galaxy for studying this kind of phenomenon, including the so-called ``formation by merger'' scenario for metal-rich GCs (Schweizer 1987; Ashman & Zepf 1992). These galaxies, although they have reached a substantial dynamical equilibrium, are indeed still young enough to keep track of the past interaction both morphological and evolutionary.

Indeed, though the most exhaustive, up-to-date study of the age, metalicity and $\alpha$-enhancement gradients within NGC 3610 of Howell et al. (2004) does not find significant stellar population gradients in the outer parts of the galaxy ( $r\geq0.75~r_{\rm e}$), a clear indication of the presence of a central young stellar population in NGC 3610 has been pointed out by Silva & Bothun (1998) on the basis of the strong central H$\beta$ absorption in the optical and the IR excess $\Delta$(H-K) between ( $r \leq 0.5$ kpc = 0.25 $r_{\rm e}$) and the outer annulus ( $1.0 \leq r \leq 1.5$ kpc) induced by intermediate-age AGB stars. Silva & Bothun (1998) refer also to a central, subarcsec UV HST/FOS spectrum of the galaxy, which they put forward as a proof of intermediate-age nuclear stars, which is well-matched by a mid-F main-sequence stellar population very similar to that of M 32.

It is serendipitous that the existing (archive), space-borne ultraviolet data also include a good-quality UV spectrum of the galaxy through the large ( $10\times20''$) IUE/LWP aperture, giving the luminosity-averaged spectral energy distribution (SED) within an aperture photometrically equivalent to an aperture 7'' in radius (cf. Burstein et al. 1988). Because the two UV spectra appear quite different (see Fig. 1) especially as far as the region of the absorption lines formed by the Mg II 2800 Å, Mg I 2852 Å and the Fe I + II + Cr I feature at 2750 Å (the so-called UV triplet, well-known for showing rapid changes in relative strengths from late B to late F stars) is concerned, one can expect that their comparison represents yet more proof of the different nature of the galaxy's inner and large-aperture stellar population.

\begin{figure} \par\includegraphics[width=12cm,height=12cm]{14429fg1.eps} \vspace*{4mm} \end{figure} Figure 1:

Comparison of the IUE /LWP (10 $'' \times 20''$) spectrum of NGC 3610 and its $\sim $1'' circular central HST/FOS G270H spectrum. Note the outstanding difference of the mid-UV triplet between the two spectra of the same object. All plots are given in log F$_\lambda $+C. The HST/FOS G270H spectrum was properly smoothed to match the IUE/LWP spectral resolution (about 6-7 Å about $\lambda $2700 Å). An arbitrary shift was finally applied for showing purposes.

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2 UV observations

A good-quality (15 300 s exposure time) IUE/LWP spectrum of NGC 3610 has been obtained in 1992 at GSFC by LB. The orientation of the IUE large aperture was PA = 154$^{\circ}$. As stressed above its ``oval'' (10 $'' \times 20''$) shape is photometrically equivalent to a 7'' radius circular aperture (cf. Burstein et al. 1988), i.e. $\sim $0.5 $r_{\rm e}$ NGC 3610 (Idiart et al. 2003). As NGC 5018 (Bertola et al. 1993), an IUE/SWP spectrum shortward of $\lambda $1800 Å provided no signal, thus implying that also NGC 3610 lacks the prominent UV-upturn typical of old, metal-rich giant spheroids.

Almost simultaneously NGC 3610 was observed with the HST/FOS on 1992 November 30. A FOS circular aperture 0 $\hbox{$.\!\!^{\prime\prime}$ }$86 diameter has been used. The target acquisition procedure involved measuring the flux on a $4\times4$ grid at 0 $\hbox{$.\!\!^{\prime\prime}$ }$25 intervals so as to center the galaxy to an accuracy of 0 $\hbox{$.\!\!^{\prime\prime}$ }$25. The datum We use here is the mid-UV FOS/G270H spectrum.

The properly extracted, redshift-corrected IUE and HST/FOS mid-UV spectra of NGC 3610 are shown on a logarithmic scale in Fig. 1 (they are properly shifted for showing purposes) after a proper resolution match of the original data. Following both Burstein & Heiles (1984) and Schlegel et al. (1998), NGC 3610 is galactic extinction-free and no correction was applied. A quantitative comparison of commonly used mid-UV indices is instead given in Table 1 for both apertures. The same set of indices is given also for the center of M 32 for comparison with an intermediate-age population.

NGC 3610 has not been observed by the GALEX satellite yet, which renders a useful comparison of its optical metalicity gradient with an extended ultraviolet color (FUV-NUV) profile impossible.

Table 1:   IUE vs. FOS Mid-UV indices.

3 Mid-UV indices as a population diagnostic tool

From Fig. 1 it is evident that the FOS (central) spectrum shows a much deeper mid-UV triplet close to $\lambda $2800 Å, which reflects in the indices of Table 1 that are computed following the definition of Chavez et al. (2007). As far as the index measurements are concerned, it is well known that IUE cameras (and thus extracted spectra) are potentially affected by the camera reseau marks. We were able to verify that the LWP camera artifacts do not affect the mid-UV indices we used of in this paper (from Fig. 2 one can see that neither the red bandpass of the index FeI 3000 (ending at $\lambda $3051 Å) turns out to be influenced).

A careful analysis of the indices listed in Table 1 does suggest the following picture for the chemistry/age pattern of the center and bulk of NGC 3610, as well as for the comparison object M 32: (i) on the basis of the doubling of the main iron and magnesium indices such as FeI+II+CrI, FeI 3000, MgII 2800 and MgI 2852 the central regions (within the FOS aperture) of NGC 3610 appear systematically more rich in metals (when compared with the large IUE region) and - assuming that the metalicity of the (central) population is approximately solar (cf. Fig. 4 in Howell et al. 2004) - younger than 4 Gyr (cf. Fig. 2 of Chavez et al. 2009); (ii) The nucleus of NGC 3610 appears quite $\alpha$-enhanced compared with that of M 32 reconsidering, e.g., the much lower value of the iron index FeI+II+CrI and the slight increase of the MgWide index (cf. Fig. 5 of Chavez et al. 2009).

\begin{figure} \par\includegraphics[width=8.9cm,height=8.9cm]{14429fg2.ps} \vspace*{5mm} \end{figure} Figure 2:

IUE /LWP10105 spectrum of M 32 here for illustration (i.e. to give the right localization of the camera reseau marks). As is evident the spectral regions corresponding to the indices used here are free from reseau contamination.

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4 Comparison with NGC 5018

A similar analysis, namely IUE large aperture vs. FOS, has been adopted by Buson et al. (2004) for NGC 5018, an object with many similarities with NGC 3610 (e.g. a major merger suffered $\sim $3 Gyr ago, Leonardi & Worthey 2000). However, unlike NGC 3610, the stellar population of NGC 5018 tells a simple story, in the sense that it shows both a strict match of its IUE and HST/FOS circumnuclear spectrum and a very shallow metalicity gradient, as traced by Mg2 index (d Mg2/d  $\log r=-0.04$;Carollo & Danziger 1994). In other words, its population is very homogeneous, both in age and metal content (as is expected if the merger is largely dissipationless) and/or suffered from a rejuvenation involving the whole galaxy.

In this respect NGC 3610 could instead represent a different kind of merger, especially considering that its inner mean gradient decreases at a rate of [Z/H]=-0.30 per decade in $r/r_{\rm e}$ (Howell et al. 2004). The onset of such a gradient, though not unexpected during a dissipative galaxy merger (e.g. Barnes & Hernquist 1991; Mihos & Hernquist 1994; Kobayashi 2004), represents unequivocally a population unhomogeneity. Moreover, its stronger absorption mid-UV spectrum appears confined to its very center alone. The natural conclusion is that the central arcsec in NGC 3610 underwent significantly more rejuvenation and/or metal enrichment than its surroundings, unlike NGC 5018.

Acknowledgements
It is a pleasure to thank Sandro Bressan - largely experienced in mid-UV indices modeling - for useful discussions. I thank the anonymous referee for his/her comments improving this paper.

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All Tables

Table 1:   IUE vs. FOS Mid-UV indices.

All Figures

   \begin{figure} \par\includegraphics[width=12cm,height=12cm]{14429fg1.eps} \vspace*{4mm} \end{figure} Figure 1:

Comparison of the IUE /LWP (10 $'' \times 20''$) spectrum of NGC 3610 and its $\sim $1'' circular central HST/FOS G270H spectrum. Note the outstanding difference of the mid-UV triplet between the two spectra of the same object. All plots are given in log F$_\lambda $+C. The HST/FOS G270H spectrum was properly smoothed to match the IUE/LWP spectral resolution (about 6-7 Å about $\lambda $2700 Å). An arbitrary shift was finally applied for showing purposes.

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In the text

   \begin{figure} \par\includegraphics[width=8.9cm,height=8.9cm]{14429fg2.ps} \vspace*{5mm} \end{figure} Figure 2:

IUE /LWP10105 spectrum of M 32 here for illustration (i.e. to give the right localization of the camera reseau marks). As is evident the spectral regions corresponding to the indices used here are free from reseau contamination.

Open with DEXTER

In the text

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